We report on efforts to mimic the wetting behaviour of surfaces or leaves of certain plants, which are rendered ultrahydrophobic through a dense layer of hairs grown on top of the leaf. We use a simple moulding approach to obtain elastic hydrophilic hydrogel networks with pillar structures that may serve as model systems for such hairy surfaces. In order to generate such structures, we first generate either a steel master or directly use a lady's mantle leaf. Second, the master is moulded against a silicone to yield an elastomer, which is a negative of the hairy surface. A subsequent radical polymerization in the negative leads to the formation of an elastic hydrogel even for the very high aspect ratios characteristic of the natural system. The results of some preliminary contact angle measurements on the obtained structures are discussed.

We describe recent investigations on the impact behaviour of liquid drops onto chemically structured surfaces. The surface patterns were prepared via photochemical attachment of polymer molecules with different hydrophilicities using self-assembled monolayers of benzophenone bearing silanes. Immobilization of the polymer monolayers was followed by an ablation process to generate a chemical surface pattern. Impact experiments on systems consisting of very hydrophobic poly(perfluoroalkylethyl)acrylate coatings and hydrophilic areas show that within certain limitations the water drop has a strong tendency to reach the hydrophilic spots, even for inclined substrates. Impact experiments of drops on arrays of hydrophilic spots on the background of a perfluorinated polymer show that the drops spontaneously self-centre on the lithographically generated pattern. The obtained results suggest that the process can be used to circumvent some of the current problems in micro-array fabrication.

The normal and inclined impact of liquid drops onto chemically structured surfaces has been studied experimentally and theoretically. The surface functionalization comprised a self-assembly process of a covalently bound monochlorosilane on a silicon substrate, followed by a photochemical attachment of a polymer of defined hydrophilicity and a subsequent deep UV ablation step to create a local spot of high wettability in a region of low wettability so that a self-centring effect of the impacting liquid could be achieved.Experimentally the impact is observed using a high-speed camera, changing the impact velocity, the impact displacement from the wettable spot and the inclination of the surface. The temporal spread of the drop was recorded, yielding also the macroscopic dynamic contact angle as a function of time. A theoretical model of the drop impact is developed, based on a mass balance and on a momentum balance which includes capillary forces and viscous drag, and which accounts for the inertial and wettability effects. The theoretical predictions for the time evolution of the drop edges agree well with the experimental data.

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